I doubt this will be distinct between roles, rather that it will be possible for all level 10s to get one no matter what the role or big battle selected. That doesnt leave me terribly encouraged for the challenge factor.
These medals are not dependent on level - a level 10 can earn a platinum medal by putting in the effort and really mastering his tasks.
At least skirmishes all came in all sizes. I'll be interested to see just how distinct the spaces are based on size (because they're apparently built from the ground up for each size configuration)
The Helm's Deep medals are a new progression currency to be exchanged for specific rewards. And the instances can be completed solo, or in 2, 3, 6 or 12-man groups, depending on the instance. Not all instances come in all sizes.
Crell-L85-Champion - Riddermark ; Swego-L85-Burglar ; Kotvi-L95-Runekeeper
Delego-L85 Hunter ; Stodden-L51-Captain ; Edhul-L61-Loremaster
Deglorion - SoA XP Disabler